What covers did you get? Mag-Hytec or PML? The Mag-Hytec rear cover almost doubles (8qt) the oil capacity, the PML is 1 qt more.
Stock oil amounts are 4. 25qts for the rear, 3. 25 qts for the front.
I have PML and the capacities are 5. 25qt rear and 3. 5qt front.
Don't reuse the old oil for the following reasons:
1) it will have some amount of metal in it because your truck is new.
2) how would you get the oil back into the diffs? What a pain compared to the new oil bottles with the funnel top.
Make sure you get synthetic oil, that is whats recommended, 75W-90.